Adding the following.
Mastodon is licensed under GNU AGPL, a free software license. Free refers to the freedom not the monetary value.
It doesn't prohibit any from selling the software with modifications made ( should provide the modified source code to whom you sell ) or setting it up on a server - providing it as a service and charge a fee.
Always be mindful of that when using the ad free, donation run, no fees charged instance you use.
Server run by the main developers of the project It is not focused on any particular niche interest - everyone is welcome as long as you follow our code of conduct!